1. Field of the Invention
This invention relates to a process of dividing a stream of metal particles differing in chemical composition into partial streams of metal particles having the same chemical composition in that the particles are singled and irradiated with electromagnetic waves, the reflected radiation is analyzed and a sorting device is controlled to change the path for each metal particle in dependence on the results of the analysis.
2. Discussion of Prior Art
Particles differing in chemical composition can be sorted by a number of known processes in which a stream of singled particles is moved past a radiant source and is irradiated and the reflected radiation is measured and used to identify the kind of particle.
In a process proposed by U.S. Pat. No. 4,212,397, diamonds are separated from the accompanying materials, a fluoresence of the stream of particles is excited by X-rays, the decay time is measured photoelectrically, and the results are delivered to a sorting logic for controlling mechanical or pneumatic diverting means.
German Offenlegungsschrift No. 30 47 536 discloses a process in which particles derived from scrap and consisting of various metals and of various alloys are exposed to X-rays or isotope radiation and the reflected radiation is measured and used to identification.
A similar process of sorting metal scrap is known from New Scientist, 63, (1982), page 490.
Engineering and Mining Journal 183, (1982, 8) describes on pages 72 to 75 a process for enriching ores, which are irradiated from two radio-active sources having different energy levels, whereafter the scattered gamma radiation is used to distinguish the particles.
The processes mentioned above can be used in part only for certain kinds of particles or have the basic disadvantage that they cannot be used in a field which is increasing in significance, namely, the recovery of the nonferrous metal fractions of scrap. This is due to the fact that most of the particles obtained in that case have been subjected to a surface treatment or provided with a surface coating so that the results of measurement are distorted and, as a result, the yield is not satisfactory.
For this reason, it is an object of this invention to provide a sorting process which is based on the principle of the known process and in which the surface finish of the metal particles is no longer significant, i.e., a process for separating metal particles regardless of their surface finish. In other words, a process is desired in which the nature of the core of each particle is detected and the particles can be sorted with high qualitative and quantitative yields.
In a process of the kind described first hereinbefore, this object is accomplished in that the melting point and/or the melting temperature range of the metal particle is used as a criterion for the distinction between the metal particles. For this purpose, the metal particles are spot-fused by means of a laser beam and the temperature profile is measured by means of a radiation pyrometer and is electronically analyzed. The melting temperature range is suitably used as a criterion for a fine sorting of alloys which contain the same base metal. A predetermined temperature interval or the energy consumption of the laser can be used as a criterion in determining the melting temperature range. The process according to the invention is preferably applied to shredded scrap from which non-metallic and ferromagnetic constituents have already been separated.
A radiation pyrometer integrally combined with a laser is suitable used to carry out the process. Such a system has been described in Temperature 5 (1982) on pages 439 to 446. On the surface of each particle a spot which is, e.g., 1 mm in diameter is heated to fusion by a pulsed laser beam and the temperature is measured at the same time. The melting point or the lower end of the melting temperature range has been reached when the temperature does not rise further in spite of an additional supply of energy. The temperature which is then measured can be used to identify the particle.
By such systems, the melting and the measurement can be effected within a fraction of a second so that the measurement is not significantly affected by the movement of the particle. Such processes can be considered as being economical when at least 10 particles can be identified per second. Singling and sorting means operating at high speed must be used for that purpose.
The economy of the process can be increased further in that the velocity of the stream of particles is increased because stationary testing means are replaced by testing means which move with the stream of particles at least for the duration of each measurement so that the relative velocity between the testing means and the object being tested is zero or negligibly small. If the stream of particles advances along a straight line, the testing device can be reciprocated along a straight line so that a new particle is tested during each forward stroke, or one or more testing devices may revolve around an orbit in synchronism with the stream of the singled particles.